This invention relates generally to a probe for measuring the concentration of oxygen in fluids. More particularly, the invention relates to a probe having a sensor tip defining a solid electrolyte which is in contact on one side with a known reference fluid and on the opposite side with the fluid whose oxygen content is to be measured. In such a probe, a voltage is generated across conductors on the two sides of the electrolyte as oxygen ions conduct through the electrolyte. The magnitude of the voltage is a function of the temperature of the electrolyte and of the log of the ratio of the oxygen partial pressures on the two sides of the electrolyte.
An oxygen-measuring probe of this general type is disclosed in Bannister et al U.S. Pat. No. 4,193,857. In that probe, a sensor tip formed of a solid electrolyte material is concentric with one end portion of an elongated tube and is secured to the tube by an hermetic seal which isolates the reference fluid from the fluid being measured.
Difficulty is encountered in producing an hermetic seal which can withstand rapid and repeated cycling through a wide range of temperatures. In an effort to produce a reliable seal, Bannister et al modify a conventional solid electrolyte material with another material to form a composite sensor tip which purportedly lends itself to being sealed in an alumina tube. This construction, however, results in an adulterated sensor tip which can lead to imprecise sensing and particularly at lower temperatures where the resistance to ion flow through the sensor tip is relatively high. In spite of the adulterated composition of the sensor tip, cracking still can occur at the seal between the sensor tip and the alumina tube. Moreover, the sensor tip is relatively expensive because of the need to modify the composition of a conventional sensing tip which can be purchased on the open market.